For optimum benefit to occupant safety, side airbags used in vehicles should deploy very rapidly. Arranged in a sealed bag, for example in the backrest of the seat assigned thereto, side airbags must emerge from the stowed position within a few milliseconds of a side impact and inflate between the seat/occupant and bodywork side on which the side impact takes place. For producing the gas, therefore, a gas generator or inflator is provided which produces the gas which inflates the airbag, wherein the inflator is generally arranged inside the airbag. So that during the side impact the airbag is rapidly emptied, a vent is configured as a simple opening. Here there is the possibility that the emptying may take place too rapidly, i.e. that the gas does not remain sufficiently long in the airbag, so as to retain the internal pressure.